Acetic acid is commercially produced by methanol carbonylation. Prior to 1970, acetic acid was made using a cobalt catalyst. A rhodium carbonyl iodide catalyst was developed in 1970 by Monsanto. The rhodium catalyst is considerably more active than the cobalt catalyst, which allows lower reaction pressure and temperature. Most importantly, the rhodium catalyst gives high selectivity to acetic acid.
One problem associated with the original Monsanto process is that a large amount of water (about 14%) is needed to produce hydrogen in the reactor via the water-gas shift reaction (CO+H2OCO2+H2). Water and hydrogen are needed to react with precipitated Rh(III) and inactive Rh(III) species to regenerate the active Rh(I) catalyst. This large amount of water increases the amount of hydrogen iodide, which is highly corrosive and leads to engineering problems. Further, removing a large amount of water from the acetic acid product is costly.
In the late '70s, Celanese modified the carbonylation process by adding lithium iodide salt to the carbonylation. Lithium iodide salt increases the catalyst stability by minimizing the side reactions that produce inactive Rh(III) species and therefore the amount of water needed is reduced. However, the high concentration of lithium iodide salt promotes stress crack corrosion of the reactor vessels. Furthermore, the use of iodide salts increases the iodide impurities in the acetic acid product.
In the early '90s, Millennium Petrochemicals developed a new rhodium carbonylation catalyst system that does not use iodide salt. The catalyst system uses a pentavalent Group VA oxide such as triphenylphosphine oxide as a catalyst stabilizer. The Millennium catalyst system not only reduces the amount of water needed but also increases the carbonylation rate and acetic acid yield. See U.S. Pat. No. 5,817,869.
In acetic acid processes measurement of the various components in the system has been used for controlling and monitoring the process. For example, U.S. Pat. No. 6,552,221 teaches process control for acetic acid manufacture using infrared spectroscopy. According to the '221 patent, samples are collected from columns and/or transfer lines downstream of a reactor vessel, and the concentration of one or more components in the sample is measured by an infrared analyzer. The concentration measurements are then used to make adjustments in the concentration of components in the reaction system, directly or indirectly, such as by adjusting the temperature profile in a particular column, the flow rate of solution in to or out of a column, the vent gas rate out of the reactor or a column, or the addition or extraction of a component to or from the solution. The components measured include water, acetic acid, methyl acetate, methyl iodide, aldehydes, hydrocarbons, propionic acid, and hydrogen iodide. Similarly, U.S. Pat. No. 6,362,366 teaches an online method to measure components in the reactor mixture.
Several drawbacks regarding the use of infrared spectroscopy with acetic acid processes have been found as disclosed herein, including large H2O and acetic acid absorbance values while having relatively weak MeI absorbance values. While large absorbances can be helpful in quantitation, in the acetic acid process, the absorbance values of H2O and acetic acid are so large that they have a tendency to overlap and interfere with absorbances of other components thereby making quantitation difficult. Additionally, a large number of calibration standards are required for acetic acid processes to obtain calibration models with acceptable accuracy. Moreover, infrared has drawbacks regarding hardware flexibility and implementation in a process environment.
There have been numerous infrared based studies carried out with regard to methanol (MeOH) carbonylation. In other processes, Raman spectroscopy has been employed. For example U.S. Pat. No. 6,100,975 used Raman spectroscopy for analyzing fluid streams containing petroleum products. However, Raman spectroscopy has not been heretofore disclosed by academic or patent literature for use in acetic acid processes, except in pending application Ser. No. 12/906,575, filed Oct. 18, 2010. Application Ser. No. 12/906,575 employs Raman spectroscopy to measure the concentration of a component in an acetic acid reaction mixture.
Similar to infrared, Raman is a form of vibrational spectroscopy. However, infrared bands are associated with light absorption and arise from a change in the dipole moment of a molecule. In contrast, Raman bands are associated with light scattering and arise from a change in polarizability of a molecule. While some molecules exhibit only Raman or only infrared vibrational transitions, others exhibit both Raman and infrared bands but with different intensities and selectivity. Accordingly, in an acetic acid process, it is unknown whether the components in the process will provide sufficiently observable spectra to determine concentration, and furthermore, in a multi-component system, some spectra may overlap therefore making observation difficult. Therefore, it is unknown whether Raman spectroscopy can be employed in a multi-component system such as an acetic acid production process to accurately measure various components.
What is needed is a system that is easily employable in an acetic acid process, which permits accurate measurement of the components without burdensome calibration standards.